1. Field of the Invention
The present invention relates to a rolling tool for both expanding and reforming the end of a tube having an off-round cross-section. It is particularly useful in reforming and expanding dented tube ends mounted around the periphery of the tubesheet of a nuclear steam generator.
2. Description of the Prior Art
Tools for expanding the ends of metallic tubes are known in the prior art. Generally, such tools include a rotatable roller cage which holds a plurality of radially extendable rollers mounted within pitched slots, and a rotatable, tapered mandrel which is slidably movable within a centrally disposed bore in the cage. In operation, the roller cage is inserted into the open end of the tube, which in turn pushes the rollers into a retracted position within the cage. Next, the tapered mandrel is simultaneously rotated and biased toward the interior of the tube, which radially extends the rollers (which are somewhat loosely held within tapered slots in the cage body) until they engage the interior surface of the tube to be expanded. Once the rollers are radially extended into such engagement, the outer surfaces of the rollers become simultaneously frictionally engaged against the inner walls of the tube, while the inner surfaces of these rollers become frictionally engaged against the rotating tapered mandrel. Such frictional engagement allows the tapered mandrel to simultaneously rotate and orbit the rollers around the axis of rotation of the mandrel. Because the slots which receive these rollers are pitched at a small angle relative to the axis of rotation in the mandrel, the rotation and the orbiting of the rollers draws the mandrel within the open end of the tube in what may be generally described as a screwing or feeding motion, which in turn radially extends the rollers further and expands the end of the tube.
Such tools have been useful in the past for eliminating potential maintenace problems associated with the heat exchange tubes in nuclear steam generators. However, in order to understand the precise utility of such tools in this context, some basic understanding of the structure and function of a nuclear steam generator is necessary.
Nuclear steam generators generally comprise a secondary side, a tubesheet, and a primary side which circulates water heated from a nuclear reactor. An example of such a generator is disclosed in U.S. Pat. No. 4,262,402, assigned to Westinghouse Electric Company, the assignee of the present invention. The secondary side of the generator includes a plurality of U-shaped tubes, as well as an inlet for admitting a flow of non-radioactive feed water. The inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in a tubesheet which hydraulically separates the primary side of the generator from the secondary side. The primary side in turn includes a divider sheet which hydraulically isolates the inlet ends of the U-shaped tubes from the outlet ends. Hot, radioactive water flowing from the nuclear reactor is forceably pumped through the primary side of the generator (which is bowl-shaped) into the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets, up to the tubesheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. The water transfers its heat through the walls of the U-shaped tubes to the non-radioactive feed water flowing through the secondary side of the generator, thereby converting this feed water into non-radioactive steam which in turn powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tubesheet, through the outlets of the U-shaped tubes, and into the outlet section of the primary side, where it is recirculated back to the nuclear reactor.
The walls of the heat exchange tubes in such nuclear steam generators can suffer any number of different forms of corrosion degradation, including denting, stress corrosion cracking, intragranular attack, and pitting. In situ examination of the tubes within these generators has revealed that most of the corrosion degradation occurs in what are known as the crevice regions of the generator. The principal crevice region for each of the U-shaped tubes is the annular space between the heat exchange tube and the bore in the tubesheet through which the tube extends. Corrosive sludge tends to collect within this crevice from the effects of gravity. Moreover, the relatively poor hyraulic circulation of the water in this region tends to maintain the sludge in this annular crevice, and to create localized "hot spots" in the tubes adjacent the sludge. The heat radiating from these "hot spots" acts as a powerful catalyst in causing the exterior walls of the heat exchange tubes to chemically combine with the corrosive chemicals in the sludge.
In order to prevent such corrosion and tube cracking from occurring in the annular crevices surrounding the tubes in the tubesheet, prior art rolling tools have been used to radially expand the ends of these tubes extending through the tubesheet. Such tube expansions eliminate the annular space between the bores of the tubesheet, and the heat exchange tubes. While such prior art tools are capable of effectively expanding the ends of undamaged tubes in the central portion of the tubesheet, such tools are difficult (if not impossible) to use in tube ends which have been dented into an off-round shape by hard bits of debris (such as nuts, bolts, cover plates, etc.) which have inadvertently found their way into the fast-flowing water of the primary system of the generator. Such dents around the ends of the heat exchange tubes prohibit the insertion of a roller cage having a diameter large enough to effectively expand the tube against its respective bore in the tubesheet. If the operator of such a tool uses a cage of a smaller diameter in an attempt to reform the shape of the tube into a circular cross-section, the rollers of the case may become "stalled" when the roller-engaging mandrel is extended and rotated due to the fact that the off-center shape of the interior of the tube may prevent some (if not all) of the rollers from frictionally engaging the inner tube wall when the mandrel is rotated. If the rollers of the cage do not frictionally engage the inner walls of the tube when the mandrel is extended and rotated, the rollers cannot orbit against the inner wall in a tube-reforming motion. Such a failure of the tool to reform the tube end would not only prevent the rolling tool from eliminating the annular space between the outer wall of the tube and the bore which surrounds it in the tubesheet, but would also prevent other tube maintenance devices (such as eddy current probes, sleeving tools, rotopeening spindles and plugs) from being inserted into the open end of the tube. Still another shortcoming of such prior art rolling tools is the difficulty in using them to roll or reform the peripherally-located tube ends around the outer edges of the tubesheet. Such tube ends are located directly adjacent the inside wall of the bowl-shaped primary side of the nuclear steam generator, and it is difficult, if not impossible, to manipulate some tools in these area. Finally, some of these rolling tools are non-automatic, which in turn necessitates exposing a human operator to the potentially dangerous radioactivity existing within the primary side of the generator.
Clearly, there is a need for a tool cabable of reliably reforming and expanding a dented tube end in the tubesheet of a nuclear steam generator. Ideally, such a tool should be remotely controllable, and fully cabable of easily and conveniently reforming and expanding the ends of tubes located in the peripheral regions of the tubesheet of the generator.